The present invention relates to techniques for providing visual inspection of samples that have analytical radiation incident thereon.
When making a sample measurement either in transmission or in reflectance, it is important to know the exact portion of the sample exposed to the measuring beam. This is especially critical where high spatial resolution is required, as for example in an optical microscope. In the case of measurements made through a microscope in particular, the means by which the desired area of the sample is identified and positioned for measurement depends, in general, on the ability of the operator to see the entire sample, either directly in the microscope eyepiece or indirectly on a television monitor screen or other remote viewing device. For this purpose, such microscopes are provided with an illuminator whose radiation is in the visible portion of the optical spectrum, and are designed to transmit that radiation to the operator along a path which assures virtual coincidence with the path of the analytical radiation, which may be from a region of the spectrum which is not visible.
In the general case, the sample area of interest is smaller than the field of view of the measuring system, and is mounted on a support whose optical characteristics are different from those of the sample itself, or are unknown. It is therefore desirable that the measured region be restricted to the sample alone. However, the sample area may be quite small in both absolute and relative terms. Since the signal-to-noise ratio of the measurement is proportional to the area of the sample measured (because the noise is independent of the sample area while the signal is directly proportional to it), it is desirable that all of the sample be included.
A typical prior art approach is to place an adjustable mask in a viewable image plane. The mask typically includes a number of opaque elements (typically blackened metal sheets) that can be moved in the image plane relative to the image of the sample so as to block selected portions of the field of view. In use, the area of sample to be measured is centered in the measuring field of view by visual means, and the mask is adjusted, also by visual means, to just exclude all the non-sample background.
The mask may be of the adjustable diaphragm type whose geometry is restricted to a circle of adjustable radius, in general greater than zero, although double-diaphragm designs can provide extinction. Alternatively, the mask may be a four-blade device in which pairs of opposed blades are arranged orthogonally in parallel displaced planes. This device is more generally useful, because it will go to complete extinction, and its rectangular geometry can be adjusted to more nearly match the geometry of most samples, which are not usually circular in projection.
Unfortunately, it is not always easy to adjust the mask to be just tangent to the side of a sample. This is a particular problem if the sample is dark, in which case, it may be almost impossible to determine the point at which the mask is just tangent to the sample.
Several prior art microscope designs, such as that of Horiba, partially solve the problem by providing a separate visual pathway around the mask. The visual images of the sample both through and around the masking device are combined in the eyepiece focal plane by means of a complex arrangement of beamsplitters and mirrors, while the measurement beam is restricted to pass through the mask assembly only. This solution allows the operator to see the open region of the mask superimposed on the sample, and should, in principle permit exact adjustment of the mask to tangency with the outline of the desired sample area. However, in practice it is not possible to align all of these extra optics, some of which must be moved out of the way of the measuring beam, so that perfect virtual coincidence with the measuring beam is assured. As a result, even though the mask appears to be perfectly aligned with the sample, either some sample area is excluded from the measuring beam, or some background is included in the measuring beam.
Thus, the prior art provides a choice between a relatively straightforward system that may be difficult or impossible for the user to adjust, and a more complicated and expensive system that is apparently easy for the user to adjust, but may give spurious results.